This invention relates generally to a process and apparatus for producing electrically charged nonwoven webs. In one aspect, the invention relates to the manufacture of electrically charged meltblown fibrous materials in which the charge is applied during the meltblowing process. In another aspect, the invention relates to electrically charging meltblown fibers using a collector apparatus with an electrically charged surface.
Meltblowing is a one step process in which a molten thermoplastic is extruded to form a plurality of fibers. Converging sheets of high velocity air blow the fibers onto a collector surface where they are entangled and collected forming a nonwoven web. These webs, called meltblown nonwoven fabrics, have excellent properties for many uses, one of which is filtration of gases and liquids.
The microscopic diameters of the entangled fibers of the meltblown web are ideally suited for filtering finely divided particles out of a gaseous or liquid media. The filtration efficiency of these nonwoven materials can be improved by applying a persistent electrostatic charge to the fibers. The charged webs are frequently called electrets. U.S. Patents which disclose nonwoven fibrous electrets include U.S. Pat. Nos. 4,215,682, 4,375,718, 4,588,537, 4,592,815. A process for applying the electric charge to the molten or hot fibers during the meltblowing process is disclosed in U.S. Pat. No. 4,215,682. The electrostatic charging of the fibers in the hot or molten state of the polymer permits the charges to migrate into the polymer(since its electrical resistance is lower then) and remain trapped upon cooling or crystallizing of the polymer. This increases the charge life of the electret.
In the process disclosed in U.S. Pat. No. 4,215,682 the charging is achieved by passing the extruded fibers through an intense electrostatic field. The electrostatic field, or corona zone, is established near the die tip using high voltage electrodes. The charged fibers are collected on a collector screen for forming the web.
Collection of the charged fibers on an electrically conductive and grounded screen can cause the fibers to lose their charge and can cause the fibers to pack more densely. Loss of charge may also lower the filtration efficiency.